Whoa, this is good stuff. +1 for the awesome, detailed explanations.
Haven't really thought too much about them though, though I probably should have considering how big of a part they are to the story, lmao.
I figured since we know they're a highly evolved species because of the mass relays, etc, they're there to make sure no other species will ever out-evolve them. Since they have that certain cycle they go about for when they'll return, they probably figure within that time period no other species should out power, out evolve them, and that they can stop them before that happens. Since us humans are rapidly evolving through technology and such, it's pretty obvious why they'd target us next.
Credit to .Impact for this good lookin' guy right here<3 ∞